Convergent link system for connecting a screed to the traction unit of a paving machine

ABSTRACT

An upper pair of arms is pivotally connected at its forward and rearward ends to the screed and to the rear portion of the traction unit. A lower pair of arms is pivotally connected at its forward and rearward ends to the screed and to the rear portion of the traction unit. Imaginary extensions of the upper and lower pairs of arms converge and intersect at points defining a transversely extending effective tow axis, which axis is disposed adjacent the subgrade and forwardly of the rearwardmost extremity of the traction means supporting the traction unit.

United States Patent [191 Smith et al.

[ June 24, 1975 CONVERGENT LINK SYSTEM FOR CONNECTING A SCREED TO THE TRACTION UNIT OF A PAVING MACHINE [75] Inventors: Fred T. Smith, Dearborn Heights,

Mich; Julian M. Munyon, Big Rock; James J. Gebhardt, Jr., Lisle, both of 111.

[73] Assignee: Barber-Greene Company, Aurora,

[22] Filed: Apr. 30, 1973 [21] App]. No.: 355,931

[52] US. Cl 404/84; 404/118 [51] Int. Cl. E0lc 19/00 [58] Field of Search 404/101, 108, 118, 84,

[56] References Cited UNITED STATES PATENTS 2,453,510 Jackson 404/114 2,922,345 1/1960 Mentes 404/84 3,029,716 4/1962 Shea 404/84 3,054,334 9/1962 Barber 404/108 3,299,642 1/1967 Feiner 404/108 X 3,403,609 10/1968 Bradshaw 404/118 X 3,559,543 2/1971 Schwoebe1.... 404/101 3,566,759 3/1971 Gurries.-. 404/84 Primary Examiner-Ni1e C. Byers, Jr.

[57] ABSTRACT An upper pair of arms is pivotally connected at its forward and rearward ends to the screed and to the rear portion of the traction unit. A lower pair of arms is pivotally connected at its forward and rearward ends to the screed and to the rear portion of the traction unit. Imaginary extensions of the upper and lower pairs of arms converge and intersect at points defining a transversely extending effective tow axis, which axis is disposed adjacent the subgrade and forwardly of the rearwardmost extremity of the traction means supporting the traction unit.

7 Claims, 5 Drawing Figures PATENTEDJuu 24 1975 SHEET CONVERGENT LINK SYSTEM FOR C(INNECTING A SCREED TO THE TRACTION UNIT OF A PAVING MACHINE BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to paving machines of the type having a traction unit and. a screed which is towed by the traction unit. More particularly, the present invention relates to the means for connecting the screed to the traction unit of such a paving machine.

2. The Prior Art As is known to those skilled in the art, the conventional means for connecting a screed to the traction unit of a paving machine includes the use of a pair of longitudinally extending draft arms disposed one at each side of the traction unit. The rear ends of these draft arms are connected to the screed. The forward ends of the draft arms are usually connected to the frame of the traction unit by vertically sliding pivots. Power means, most often in the form of hydraulic cylinders, are connected to the forward ends of the draft arms for raising and lowering the same thereby to vary the angle of attack of the screed. A change in the angle of attack of the screed will increase or decrease the thickness of the mat or lift of asphalt thereby to provide the desired grade control.

In some instances, the forward ends of the draft arms are connected to the frame of the traction unit by fixed pivots. In this arrangement, the screed is pivoted relative to the rear ends of the draft arms and associated with suitable power means for changing the angle of attack of the screed.

In either of the arrangements just described, the draft arms arenecessarily rather massive and substantial in construction as they must be capable of transferring a considerable draft force from the traction unit to the screed and they necessarily span a considerable distance longitudinally of the paving machine. These draft arms must be of rather rigid construction in order to minimize deflection.

As is known to those skilled in the art, in order to ob tain proper response characteristics for a floating screed of the type under consideration, the pivot axes of the draft arms, which define the axis about which the screed moves, must be located a considerable distance forwardly of the screed. In other words, in order to obtain the desired response characteristics for the screed, the same must be attached to the traction unit for pivoting or swinging movement about a radius of substantial extent.

It is desirable to locate the pivot axes at the forward ends of the draft arms as low as possible to facilitate the transfer of draft forces from the traction unit to the draft arms. Such a low location of the draft arm pivot axes minimizes the development of undesirable reaction forces in the frame of the traction unit and also improves the response characteristics of the floating screed. However, as a practical mater, the forward pivot axes of the draft arms cannot be lowered to the extent desired, as a certain amount of clearance is necessary between the subgrade and the forward ends of the arms.

There are a number of other disadvantages associated with these draft arms. Such members are rather expensive since they must be of massive construction and of substantial length. Since these draft arms extend along opposite sides of the traction unit, they necessarily occupy considerable space and restrict the location of the traction means for the traction unit, which traction means may be either rubber tires or crawler-type devices.

SUMMARY AND OBJECTS OF THE INVENTION The present invention may be summarized as relating to a unique linkage assembly extending between the rear end of the traction unit and the screed, such linkage assembly constituting the sole means for transferring draft forces from the traction unit to the screed. The linkage assembly includes upper and lower sets or pairs of links or arms pivotally connected with the traction unit and the screed for pivoting movement about horizontal transverse axes. Imaginary extensions of the links converge to define an effective tow axis for the screed, this effective tow axis extending transversely of the traction unit closely adjacent the subgrade and forwardly of the rearwardmost extremity of the traction means supporting the traction unit.

A primary object of the present invention is the provision of a new and improved convergent link system for attaching the screen to the traction unit of a paving machine.

Another object of the present invention is the provision of upper and lower link assemblies pivotally engaged with the screed and with the rearward portion of the traction unit, such linkage assemblies constituting the sole members for transferring draft forces from the traction unit to the screed.

Still another object of the present invention is the provision of a linkage assembly of the type referred to t in the foregoing object, wherein imaginary extensions of the upper and lower linkage assemblies converge to define a transversely extending effective tow axis for the screed, such effective tow axis being disposed closely adjacent the subgrade and forwardly of the rearwardmost portion of the traction means supporting the traction unit.

Another object of the present invention is the provision of a linkage assembly of the type described, wherein means are provided for adjusting the pivot connection for at least one of the link assemblies thereby to vary the location of the effective tow axis.

Yet another object of the present invention is the provision of a linkage assembly of the type referred to in the foregoing objects, wherein means are provided for adjusting the horizontal position of one of the horizontal pivot axes of at least one of the link assemblies thereby to vary the angle of attack of the screed to provide grade control, constant response sensitivity of the screed being maintained during such adjustment.

Another object of the present invention is the provision of a linkage assembly of the type described, wherein adjustable pivot means are provided for connecting the forward ends of both the upper and lower linkage assemblies to the traction unit, such adjustable pivot means providing for moving of the forward ends of both the upper and lower linkage assemblies up and readily adaptable for automatic grade and slope control.

These and other objects and advantages of the present invention will become apparent from the following specification disclosing a preferred embodiment shown in the accompanying drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevational view of a paving machine embodying the present invention;

FIG. 2 is a fragmentary side elevational view of a modification of the present invention;

FIG. 3 is a diagrammatic view of a hydraulic system constituting an automatic grade and slope control system with which the present invention may be readily associated;

FIG. 4 is an enlarged, fragmentary, side elevational view of still another modification of the present invention; and

FIG. 5 is a section taken along the line 5-5 of FIG. 4.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring to FIG. 1, a self-propelled paving machine of a type well known to those skilled in the art is generally designated 10. The paving machine may be considered as including a traction unit, generally designated 11, and a screed, generally designated 12. As mentioned above, the present invention has to do with the means for attaching the screed to the traction unit for being towed by the latter.

The traction unit 11 includes a chassis 13 having a hopper 14 at the front thereof, which hopper receives the asphalt paving material. The traction unit includes a pair of flight conveyors (not shown) which communicate at their forward ends with the hopper and which communicate at the rearward ends with transversely extending augers or screw conveyors, one of such augers being illustrated and designated 15. The various conveyors and augers transfer the asphalt material 16 rearwardly from the hopper 14 so as to form a lift or mat 16a by the action of the screed 12 in a manner well known to those versed in the art.

The traction unit 11 is preferably powered by a suitable internal combustion engine for driving a pair of large rear wheels, one being illustrated and designated 17. The traction unit also includes two pairs of smaller forward wheels, one wheel of each pair being illustrated and designated 18 and 19. Of course, the traction unit may be supported and self-propelled over the existing surface, road or subgrade 20 by other forms of traction means, such as endless crawler-type tracks.

The screed 12 is illustrated in the drawings only in the end elevation. The screed extends transversely of the traction unit 1 1 for a distance at least as great as the width of the traction unit 11. Of course, in actual practice opposite ends of the screed can extend well beyond the side extremities of the traction unit. The screed 12 will preferably include heating and vibrating or tamping means (not shown) for compacting the asphalt material to form the mat or lift 16a. As this time it should be mentioned that this invention is applicable to a wide variety of known types of screeds.

The present invention relates to a linkage assembly including an upper linkage, generally designated 22. The forward extremity of this link is mounted to the frame of the chassis 13 for pivoting movement about a horizontal transverse axis 23. It is noted that the link 22 is pivotally connected to the traction unit 11 adjacent the rear end thereof at one side of the chassis 13. The pivotal connection defining the pivot axis 23 may be constituted by a pin or bolt (not shown) secured to a frame member of the chassis 13. The link 22 is preferably contained in a vertical longitudinal plane and has its rearward extremity connected to the screed 12 for pivoting movement about a horizontal transverse axis 24. A pin or bolt may constitute the pivotal engagement between the link 22 and the screed 12. The link 22 may be provided with a manually operated turn-buckle assembly 25 for varying the length of the link 22 so as to change the angle of attack of the screed as will become apparent herein.

The present invention further includes a lower link, generally designated 26. The link 26 is connected to a suitable frame member of the chassis 13 of the traction unit for pivoting moving about a horizontal transverse axis 27. Again, it is noted that the link 26 is connected to the traction unit adjacent the rear thereof and at one side of the chassis 13. The link 26 is contained in a vertical longitudinal plane and is connected at its rearward extremity to the screed 12 for pivoting movement about a horizontal transverse axis 28. Any suitable form of pin or bolt arrangement may be provided for pivotally connecting the link 26 to the traction unit and to the screed.

Apertures or bolts 30, 31 are provided on the chassis 13 for adjusting the pivot axes at the forward ends of the links 22, 26. Similarly, additional bolts or apertures 32, 33 are provided on the screed 12 for adjusting the pivot axes at the rear ends of the links 22, 26. It is apparent that such adjustment means provides for varying the angle of inclination of the links 22, 26; the purpose of such adjustment will be explained hereinbe low.

It will be understood that upper and lower links, identical to the links 22 and 26,respectively, are provided at the other side of the traction unit 11 for pivotal engagement with the latter and with the screed 12. In other words, in the embodiment of the invention illustrated in FIG. 1, the means for attaching the screed to the traction unit are symmetrical with respect to the longitudinal centerline of the paving machine 10. However, it will be understood that it iswithin the scope of the invention to substitute a single lower link for the links 26, such single link being located in a vertical plane containing the longitudinal centerline of the paving machine. Also, it is within the scope of the invention to substitute a single centrally located upper link for the two upper links 22.

An imaginary extension of the upper link 22 is designated 35 and an imaginary extension of the lower link is designated 36. As seen in FIG. 1, these extensions converge and intersect at a point 37. Of course, the imaginary extensions of the two links on the other side of the paving machine would also converge and intersect to form another point 37. These two points define a transversely extending effective tow axis which is located at the forward portion of the traction unit 11 and which is disposed reasonably close to the subgrade 20. This effective tow axis may be considered as the axis about which the screed 12 moves or responds.

Assume that the forward ends of the links 22 are adjusted so as to be engaged with the lower apertures or pins 30. In this event, the imaginary extensions 35' converge and intersect the extensions 36 at points 38 thereby defining a new effective tow axis disposed considerably rearwardly of the effective tow axis constituted by the points 37. With the links 22 positioned as just described, assume that the forward ends of the links 26 are adjusted so as to be pivotally engaged with the lower apertures or pins 31. Now, the imaginary extensions 35', 36 of the links converge and intersect at points 39 thereby locating the effective tow axis beneath the subgrade and considerably forwardly of the paving machine.

When the transverse slope of the screed 12 is the same as that of the traction unit 11, the effective tow axis will be parallel with the subgrade. Of course, when the slope of the screed is different from the slope of the tracton unit (and this is most often the case due to operation of the well known slope control adjustment mechanism) the effective tow axis will be inclined slightly with respect to the subgrade.

Referring to FIG. I, assume that the screed is attached to the traction unit by a pair of upper links 22 and by only a single lower link 26, the latter being disposed along the longitudinal center line of the paving machine. With such an arrangement, the effective tow axis would be defined by a straight line connecting three points established by intersection of the imaginary extensions of the links with a vertical transversely disposed plane containing such line. That is to say, the imaginary extensions of these three links would intersect such vertical plane at three points defining a straight line constituting the effective tow axis. Regardless of whether the upper or lower linkage is constituted by single or plural links, in each case the upper linkage and the lower linkage will converge such that imaginary extensions thereof will define an effective tow axis adjacent the subgrade and considerably forwardly of the screed.

It is apparent that by adjusting the pivot axis of the forward and rearward ends of the links 22, 26, the effective tow axis may be located in a variety of positions both longitudinally of the traction unit as well as vertically thereof. All practical locations for the effective tow axis will be well forwardly of the rearwardmost extremity of the traction means in the embodiment illustrated such traction means are constituted by the wheels 17. As is known to those skilled in the art, the axis about which the screed moves or responds must be forwardly of the rear traction means in order for the screed to operate properly. Locating the effective tow axis considerably forwardly of the rear wheels permits such rear wheels to move up and down without affecting the angle of attack of the screed and its consequent functioning. This is an important consideration because of wheel bounce, rear wheel dig-in due to wheel slippage and/or other factors inherent in wheel drives.

When the effective tow axis is located considerably forwardly of the traction unit, such as at 37, the response of the screed is consequently much slower. This slower response of the screed results in depositing more ofthe asphalt material in low areas and forming of a thinner lift over high areas. On the other hand, when the effective tow axis is moved to a more rearward position, such as at 38, the response of the screed is'much faster.

Such horizontal adjustment of the effective tow axis permits the degree of response of the screed to be varied. This is significant as in some operations a small degree of leveling or grade control is desired and close conformity with existing base undulations is sought to achieve an essentially uniform thickness of the pavement lift. This objective might be sought in surfacing an existing secondary road where pavement strength and weather protection are the prime objectives and optimum leveling too costly. On the other hand, optimum leveling may be the primary objective on primary roads, such as high speed highways.

It should also be mentioned that this adjustment of the effective tow axis or response axis of the screed permits reducing the sensitivity of the floating screed to variations in pressures against its front or leading surface. These presence variations may be caused by variations in the mix stability, forward rate of travel of the screed, or changes in the level of the mix carried ahead of the screed.

As mentioned above, the vertical location of the effective tow axis may also be varied. This factor is of significance as concerns operation of the screed. Experience and load analysis indicate that the effective pivot axis of the screed should be as near the surface of the subgrade as possible the optimum location for the effective tow axis may even be slightly below the subgrade. These low positions of the effective tow axis reduce the tendency of the screed to ride up and down as the quantity of asphalt material in front of the screed varies. For example, if the effective pivot axis of the screed were to be located at a substantial elevated position above the subgrade and the load in front of the screed increased as the result of excess material, the draft forces transferred to the screed would tend to lift the same causing it to lose control over the leveling action. It is apparent that the linkage arrangement of the present invention permits the effective tow axis to be located near and even below the subgrade if desired, in contrast with prior art arrangements'using the massive draft arms referred to above.

The arrangement of the links 22, 26 permits the screed 12 to move up and down as desired. Since these links converge, the angle of attack of the screed will decrease when the screed moves upwardly and increase when the screed moves downwardly.

Powdered means, preferably in the form of a fluid operated assembly, are provided for lifting the screed to a transport position when the paving machine is moved from one job site to the other. One of such fluid operated assemblies is illustrated and generally designated 40. This assembly includes a cylinder 41 pivotally mounted at one end thereof to a bracket 43, the latter being mounted to the frame of the traction unit 1 1. The piston rod 44 associated with this fluid cylinder is pivotally engaged to an arm 45. This arm is secured to a bracket 46 fixed to the screed. The assemblies 40 may be activated to lift the screed 12 a substantial distance above the subgrade during transport of the paving machine. It will be apparent that the screed 12 is moved closer to the traction unit 11 during such upward lifting movement. This is obviously advantageous in making the paving machine more compact and improving machine balance by reducing the overhung load during transport.

It will be observed that the aforedescribed linkage I traction means, whether such means are constituted by plural wheels or crawler-type tracks. 7

As mentioned above, the length of the upper links 22 may be varied to change the angle of attack of the screed to control the thickness of the mat or lift 16a. This change in length of the links shifts the horizontal location of one of the horizontal pivot axes defined at corresponding ends of the links. This change in length will result in a small and negligible relocation of the effective tow axis. It is apparent that the adjustable means for varying the length of the upper links 22 couldalso be provided on the lower links 26. It may be desired to provide powered means for horizontally shifting the pivot axis at corresponding ends of one of the upper or lower pairs of links to achieve automatic grade control; such an arrangement is shown in FIG. 2 wherein the parts of the invention corresponding to those already described are indicated by the prime form of numeral. I

The forward extremity of the link 22 is pivotally engaged with one end of a bell-crank 48. This bell-crank is pivotally mounted intermediate its ends, as by a pin 49. The other end of the bell-crank is pivotally engaged with one end of a piston rod 50, whichrod is associated with a fluid actuated cylinder 51. This cylinder is pivotally engaged with the frame of the chassis 13', as by means of a pin 52.

Again, the linkage system illustrated in FIG. 2 is provided for the other side of the paving machine. Or in other words, the linkage system for attaching the screed to the traction unit is symmetrical with respect to the longitudinal centerline of the paving machine. It will be apparent that actuation of the cylinder 51 will result in changing of the angle of attack of the screed 12 for varying the thickness of the mat 16a.

Another modification of the invention is illustrated in FIGS. 4 and 5. The parts of the invention disclosed in the FIGS. 4 and 5 embodiment and corresponding to the parts already described are indicated by the double prime form of numeral.

An upper link, generally designated 55, has the forward extremity thereof pivotally engaged with a bar 56, as by means of a horizontally and transversely extending pin 57. The rearward extremity of the link 55 is pivotally engaged with the screed 12", as by means of a spherical joint 58. The link 55 is formed of threadingly engaged, telescoping members 59, 60, the member 60 being rotated relative to the member 59 by means of a hand crank 61 to vary the length of the link 55. A locking collar 63 actuated by an arm 64 is provided to lock the members 59, 60 relative to each other in any position as determined by the hand crank 61.

A lower link, generally designated 65 and curved to clear the auger 15 has the forward end thereof pivotally connected with the bar 56, as by means of a hori- Zontal pivot pin 66. The rearward end of this link is pivotally engaged with the screed 12", as by means of a horizontal pin 67.

The bar 56 is mounted for vertical reciprocal movement relative to the chassis 13" of the traction unit 1 1" by means of vertically spaced assemblies of guide blocks 70, 71 and 72. The upper end of the bar 56 is pivotally engaged with an arm 73, the latter being pivotally engaged with a bell-crank 74. The bell-crank is mounted for pivoting movement about a pin 75, which pin is mounted from a bracket 76 suitably'supported from the frame of the chassis 13". The end of the bellcrank 74 remote from this pivot axis is pivotally engaged with a piston rod 77, as by means of a pin 78. This piston rod is associated with a fluid actuated cylinder 79, the latter being pivotally connected to a bracket 80 as by means of a pin 81. The bracket 80 is suitably secured to the chassis 13".

The secured 12" is of conventional construction and includes the usual strike-off plate 83 as well as a vibratory mechanism, generally designated 84. The screed 12" preferably includes a pair of longitudinally extending, horizontally spaced apart guide bars, one of such bars being illustrated and designated 85. This guide bar is secured at its rearward end to the screed 12". A side surface at the forward end of the guide bar slideably engages a complementary surface (not shown) formed on the traction unit. It will be understood that these guide bars provide lateral stability to the screed 12".

A transversely extending beam 86 has respective opposite ends thereof secured to respective guide bars 85 through shock mounts (not shown) to permit'crowning of the screed. The beam 86 provides a convenient mounting for a slope detector forming part of automatic slope control means to be referred to hereinbelow.

It will be understood that the linkage arrangement and powered adjustment means therefor as illustrated in FIGS. 4 and 5; are duplicated on the other side of the paving machine. It is apparent that actuation of the cylinder 79 will adjust, or move up and down in unison, the pivot axes at the forward ends of both of the links 55, 65 to vary the angle of attack of the screed 12" thereby to control the mat thickness. This adjustment of the forward pivot axes of the links 55, 65 in unison as just described will raise or lower the location of the effective tow axis of the screed, but produce a small but negligible change in radial distance from the screed, thereby maintaining constant leveling response sensitivity. The embodiment of the invention illustrated in FIGS. 4 and 5, is readily adaptable for use in an automatic grade and slope control system. A preferred system is disclosed in Martenson, et al. US Pat. No. 3,691,916, assigned to the assignee of this application. Such system is partially illustrated in FIG. 3 herein in diagrammatic form; reference should be had to the aforementioned Martenson et al patent for a detailed explanation of the automatic grade and slope control system.

Referring to FIG. 3, the fluid operated cylinder 79 is actuated by a proportional control valve 88 via lines 89, 90. The proportional control valve has an outlet line 91 extending to a sump 92; the proportional control valve has an inlet line 93 extending to a pump 94. This pump communicates with the sump 92 through a line 95.

The fluid operated cylinder 79a located on the other side of the traction unit 11" and corresponding to the cylinder 79, is actuated by a proportional control valve 96 via lines 97, 98. The control valve 96 communicates with the sump 92 through a line 99. A line 100 connects the pump 94 with the control valve 96.

A grade sensor 101 activates the proportional control valve 88 through appropriate circuitry 102. It will be understood that the sensor 101 is of the type having a sensor or the like engaged with a stringline to'provide grade control input information. It will be apparent that such sensor will actuate the cylinder 79 thereby to shift or vary the forward pivot axes of the links 55, 65 to provide grade control.

A slope sensor 103 is mounted on the beam 86 to detect changes in slope of the screed 12". The sensor 103 actuates the proportional control valve 96 through appropriate circuitry 104. It will be understood that the sensor 103 actuates the hydraulic cylinder 79a to vary the slope of the screed 12" for providing the desired slope of themat being formed by the paving machine.

It will be apparent that the present invention provides an improved means for connecting a screed with the traction unit of a paving machine. The linkage assembly permits the effective tow axis to be adjusted to the optimum position for a particular job requirement. The linkage assembly of the present invention is readily adaptable for both manual and automatic control over the angle of attack of the screed. Further, the present invention permits the effective tow axis to be moved horizontally for increasing or decreasing the length of the arm or radius about which the screed moves; this controls the sensitivity of the screed. That is to say, when the upper and lower linkage describe a minimum angle of convergence, which results in a maximum horizontal spacing between the screed and the effective tow axis, the sensitivity of the screed will be decreased thereby to provide a maximum leveling function. Increasing the angle of convergence of the linkage brings the effective tow axis closer to the screed thereby increasing the sensitivity and producing a pavement profile in close conformity with existing base undulations.

It is apparent that the present invention also permits adjustment of the screed for forming asphalt mats over a substantial range of thicknesses. This provision may i be readily achieved by providing a substantial number of apertures or bolts 30-33 arranged in a vertical pattern for adjusting the elevation of the screed relative to the traction unit.

We claim:

1. In a paving machine of the type having a selfpropelled traction unit and a screed which is towed by the traction unit adjacent the rear end of the latter, wherein the traction unit includes traction means engaging the existing surface for supporting and propelling the traction unit, the improvement comprising:

a. upper, longitudinally extending, link means connected at forward and rearward extremities to the rear end of the traction unit and to the screed for pivoting movement about respective, horizontally extending, transverse axes;

b. lower, longitudinally extending, link means connected at forward and rearward extremities to the rear end of the traction unit and to the screed for pivoting movement about respective, horizontally extending, transverse axes; and

c. the points of connection between said upper and lower link means and said traction unit being located only adjacent the rear portion of the latter, the means pivotally connecting the forward end of said upper link means being located lower than the means pivotally connecting the rearward end of said upper link means, and the means pivotally connecting the forward end of the lower link means being located no higher than the means pivotally connecting the rearward end of the lower link means such that i. the imaginary forward extensions of said upper and lower link means converge to define an effective tow axis for said screed, and

ii. said axis extends transversely of the traction unit 5 closely adjacent said existing surface and forwardly of the rearwardmost extremity of the traction means.

2. The improvement according to claim 1 further defined by adjustable pivot means connected to at least one of the ends of at least one of said link means for varying the angle of inclination of said one link means thereby to adjust the vertical location of the effective tow axis. l

3. The improvement according to claim 1 further defined by, adjustment means connected with at least one of said link means to vary the horizontal location of at least one of the pivot axes thereof for varying the angle of attack of the screed.

4. The improvement according to claim 1 further defined by, adjustable pivot means connecting the forward extremities of said upper and lower link means to said traction unit, which adjustable pivot means include means for shifting the forward pivot axes of said upper and lower link means up and down in unison thereby to vary the angle of attack of the screed.

5. In a paving machine of the type including a selfpropelled traction unit having traction means for supporting and propelling the paving machine over the existing surface, and a screed disposed rearwardly of the traction unit adjacent the rear end of the latter, the improvement comprising:

a. upper, longitudinally extending, link means connected at forward and rearward extremities to the rear end of the traction unit and to the screed for pivoting movement about respective, horizontally extending, transverse axes;

b. lower, longitudinally extending, link means connected at forward and rearward extremities to the rear end of the traction unit and to the screed for pivoting movement about respective, horizontally extending, transverse axes; and

c. the points of connection between said upper and lower link means and said traction unit being located only adjacent the rearwardmost portion of the latter, the means pivotally connecting the forward end of said upper link means being located lower than the means pivotally connecting the rearward end of said upper link means, and the means pivotally connecting the forward end of the lower link means being located no higher than the means pivotally connecting the rearward end of the lower link means such that i. the imaginary forward extensions of said upper and lower link means converge to define an effective tow axis for said screed, and

ii. said axis extends transversely of the traction unit closely adjacent said existing surface and forwardly of the rearwardmost extremity of the traction means, at least one of said upper and lower link means including a. pair of links which are horizontally spaced apart transversely of the traction unit and are connected to the latter adjacent respective sides thereof.

6. The improvement according to claim 5 further defined by:

a. automatic grade control means including a grade sensor and first power means activated thereby;

b. automatic slope control means including a slope sensor and second power means activated thereby;

c. at least one of said upper and lower link means having first adjustment means for varying the horizontal position of at least one of the pivot axes defined thereby;

d. means connecting said first power means to said first adjustment means;

e. second adjustment means connected to at least one of the ends of one of the links of said pair of links for varying the vertical position of said one end of said one link; and

f. other means connecting said second power means to said second adjustment means.

7. In a paving machine of the type including a selfpropelled traction unit having traction means for supporting and propelling the paving machine over the existing surface, and a screed disposed rearwardly of the traction unit adjacent the rear end of the latter, the improvement comprising:

a. upper, longitudinally extending, link means connected at forward and rearward extremities to the rear and of the traction unit and to the screed for pivoting movement about respective, horizontally extending, transverse axes;

b. lower, longitudinally extending, link means connected to forward and rearward extremities to the rear end of the traction unit and to the screed for pivoting movement about respective, horizontally extending, transverse axes;

c. the imaginery extensions of said upper and lower link means converging to define an effective tow axis extending transversely of the traction unit adjacent said existing surface and forwardly of the rearwardmost extremity of the traction means;

(1. said upper link means including a pair of links which are horizontally spaced apart transversely of the traction unit and are connected to the latter adjacent respective sides thereof;

e. said lower link means also including a pair of links which are spaced apart transversely of the paving machine and are connected to the traction unit adjacent respective sides thereof;

f. first adjustable pivot means mounted on the rear end of the traction unit adjacent one side thereof and connected to the forward ends of the adjacent upper and lower links for moving the respective pivot axes thereof up and down in unison;

g. second adjustable pivot means mounted on the rear end of the traction unit adjacent the other side thereof and connected to the forward endsof the adjacent upper and lower links for moving the respective pivot axes thereof up and down in unison;

h. automatic grade control means including a grade sensor and first powermeans activated thereby;

i. automatic slope control means including a slope sensor and second power means activated thereby;

j. means connecting said first adjustable pivot means with said first power means for being actuated thereby; and

k. other means connecting said second adjustable pivot means with said second power means for being actuated thereby. 

1. In a paving machine of the type having a self-propelled traction unit and a screed which is towed by the traction unit adjacent the rear end of the latter, wherein the traction unit includes traction means engaging the existing surface for supporting and propelling the traction unit, the improvement comprising: a. upper, longitudinally extending, link means connected at forward and rearward extremities to the rear end of the traction unit and to the screed for pivoting movement about respective, horizontally extending, transverse axes; b. lower, longitudinally extending, link means connected at forward and rearward extremities to the rear end of the traction unit and to the screed for pivoting movement about respective, horizontally extending, transverse axes; and c. the points of connection between said upper and lower link means and said traction unit being located only adjacent the rear portion of the latter, the means pivotally connecting the forward end of said upper link means being located lower than the means pivotally connecting the rearward end of said upper link means, and the means pivotally connecting the forward end of the lower link means being located no higher than the means pivotally connecting the rearward end of the lower link means such that i. the imaginary forward extensions of said upper and lower link means converge to define an effective tow axis for said screed, and ii. said axis extends transversely of the traction unit closely adjacent said existing surface and forwardly of the rearwardmost extremity of the traction means.
 2. The improvement according to claim 1 further defined by adjustable pivot means connected to at least one of the ends of at least one of said link means for varying the angle of inclination of said one link means thereby to adjust the vertical location of the effective tow axis.
 3. The improvement according to claim 1 further defined by, adjustment means connected with at least one of said link means to vary the horizontal location of at least one of the pivot axes thereof for varying the angle of attack of the screed.
 4. The improvement according to claim 1 further defined by, adjustable pivot means connecting the forward extremities of said upper and lower link means to said traction unit, which adjustable pivot means include means for shifting the forward pivot axes of said upper and lower link means up and down in unison thereby to vary the angle of attack of the screed.
 5. In a paving machine of the type including a self-propelled traction unit having traction means for supporting and propelling the paving machine over the existing surface, and a screed disposed rearwardly of the traction unit adjacent the rear end of the latter, the improvement comprising: a. upper, longitudinally extending, link means connected at forward and rearward extremities to the rear end of the traction unit and to the screed for pivoting movement about respective, horizontally extending, transverse axes; b. lower, longitudinally extending, link means connected at forward and rearward extremities to the rear end of the traction unit and to the screed for pivoting movement about respective, horizontally extending, transverse axes; and c. the points of connection between said upper and lower link means and said traction unit being located only adjacent the rearwardmost portion of the latter, the means pivotally connecting the forward end of said upper link means being located lower than the means pivotally connecting the rearward end of said upper link means, and the means pivotally connecting the forward end of the lower link means being located no higher than the means pivotally connecting the rearward end of the lower link means such that i. the imaginary forward extensions of said upper and lower link means converge to define an effective tow axis for said screed, and ii. said axis extends transversely of the traction unit closely adjacent said existing surface and forwardly of the rearwardmost extremity of the traction means, at least one of said upper and lower link means including a pair of links which are horizontally spaced apart transversely of the traction unit and are connected to the latter adjacent respective sides thereof.
 6. The improvement according to claim 5 further defined by: a. automatic grade control means including a grade sensor and first power means activated thereby; b. automatic slope control means including a slope sensor and second power means activated thereby; c. at least one of said upper and lower link means having first adjustment means for varying the horizontal position of at least one of the pivot axes defined thereby; d. means connecting said first power means to said first adjustment means; e. second adjustment means connected to at least one of the ends of one of the links of said pair of links for varying the vertical position of said one end of said one link; and f. other means connecting said second power means to said second adjustment means.
 7. In a paving machine of the type including a self-propelled traction unit having traction means for supporting and propelling the paving machine over the existing surface, and a screed disposed rearwardly of the traction unit adjacent the rear end of the latter, tHe improvement comprising: a. upper, longitudinally extending, link means connected at forward and rearward extremities to the rear and of the traction unit and to the screed for pivoting movement about respective, horizontally extending, transverse axes; b. lower, longitudinally extending, link means connected to forward and rearward extremities to the rear end of the traction unit and to the screed for pivoting movement about respective, horizontally extending, transverse axes; c. the imaginery extensions of said upper and lower link means converging to define an effective tow axis extending transversely of the traction unit adjacent said existing surface and forwardly of the rearwardmost extremity of the traction means; d. said upper link means including a pair of links which are horizontally spaced apart transversely of the traction unit and are connected to the latter adjacent respective sides thereof; e. said lower link means also including a pair of links which are spaced apart transversely of the paving machine and are connected to the traction unit adjacent respective sides thereof; f. first adjustable pivot means mounted on the rear end of the traction unit adjacent one side thereof and connected to the forward ends of the adjacent upper and lower links for moving the respective pivot axes thereof up and down in unison; g. second adjustable pivot means mounted on the rear end of the traction unit adjacent the other side thereof and connected to the forward ends of the adjacent upper and lower links for moving the respective pivot axes thereof up and down in unison; h. automatic grade control means including a grade sensor and first power means activated thereby; i. automatic slope control means including a slope sensor and second power means activated thereby; j. means connecting said first adjustable pivot means with said first power means for being actuated thereby; and k. other means connecting said second adjustable pivot means with said second power means for being actuated thereby. 